1. Field of the Invention
The present invention relates generally to a global positioning system for use in mobile communication terminals, and in particular, to a method for determining time delay by a repeater in a mobile communication network.
2. Description of the Related Art
There have been proposed several feasible systems for implementing determination of a current position of a mobile communication terminal within a mobile communication network. One of those systems, for example, would be a conventional GPS (Global Positioning System) assisted mobile communication terminal with a GPS receiver capable of receiving a GPS satellite signal used for determination of a position of the mobile terminal. Another known system may be a positioning system utilizing pilot phase difference in base station signals. Alternatively, a modified positioning system has been also proposed in the state of the art that a mobile terminal equipped with the GPS receiver is provided with a series of acquisition assistance information from a position decision server of an Assisted GPS system (AGPS) such as a Position Determination Entity (PDE), in other words, a Serving Mobile Location Center (SMLC), to thereby obtain a GPS signal for determination of position.
Among the aforementioned positioning systems, the positioning system utilizing pilot phase difference in base station signals is usually referred to as Advanced Forward Link Trilateration (AFLT) system, in which information on a time difference of arrival (TDOA) in pilot signals from base station is utilized to determine a current position of a mobile communication terminal. The AFLT positioning system may utilize a time of arrival (TOA) of a radio wave to be measured as a round trip delay (RTD) or a round trip time (RTT) in addition to the TDOA information. Alternatively, a hybrid system utilizing both of AFLT and AGPS systems has been proposed.
However, the known positioning system that utilizes such time difference of arrival (TDOA) information, that is, the pilot phase difference in the base station signals to determine a current position of a mobile communication terminal may have a drawback that the TDOA information includes a large level of bias error, due to repeaters arranged in the mobile communications system. As such, this kind of bias error will give considerable adverse effect to a determination of a position of the mobile terminal. Explaining in more detail, in situations where that a time delay is caused by repeaters in AFLT positioning system using the TDOA information, it will often make the determination of a current position of the mobile terminal substantially impossible.
Furthermore, in situations where the time delay is generated due to the repeaters in AGPS positioning system using the GPS signal, it will subsequently induce a large level of error to a clock operation in the mobile terminal kept in synchronism with a timing system in a base station of the mobile communication system. Such a clock operation error in the mobile terminal will often function to obstruct the efficient acquisition of the GPS signal in the mobile terminal using AGPS positioning system, and under the worst condition, it will sometimes make it completely impossible for the mobile terminal to receive the GPS satellite signals even in an open sky environment.
With a view to solving the aforementioned problems caused by those time delays from the repeaters in the mobile communication system, one solution has been proposed in the art in which information on those repeaters arranged in the mobile communication system is organized and utilized as a set of database. That is to say, the position decision server in AGPS positioning system, such as PDE, will retain the coordinate values of the repeaters and the time delay information of each repeater as a set of database information, so that the information is utilized for decision of a position or generation of acquisition assistance information. However, this database system has a disadvantage, it will be very difficult to organize the variety of information relative to a large number of repeaters in the mobile communication network into a set of database in most efficient manner. Furthermore, the database system still has more problems in that whenever a request for decision of a position from the mobile terminal is made, the position decision server needs to search the database for information on repeaters, and that it usually requires additional cost and time to periodically update and maintain the database on repeater information in the mobile communication network.
In the meantime, in the AGPS system in which acquisition assistance information data are received from the position decision server, a modified solution that a fixed repeater time delay is also taken into account in addition to the acquisition assistance information data has been proposed in order to prevent acquisition of the GPS satellite signals owing to the time delay caused in those repeaters from failure. This solution with consideration of such a fixed repeater time delay may at least or partially compensate for the time delay problem as mentioned above, however it still fails to overcome a disadvantage that a time duration (Time To First Fix, hereinafter referred to as “TTFF) required for acquisition of the GPS satellite signal increases as the fixed repeater time delay in AGPS system. As a result, such an increase in TTFF by fixation of the repeater time delay subsequently causes the repeater time delay to be fixed with a smaller value with a view to preventing the increase in TTFF, which may eventually lead to a total failure in acquisition of the GPS satellite signal for a mobile communication terminal using AGPS positioning system.